The present invention relates generally to voltage power supplies and more particularly to a voltage power supply which is especially suitable for providing a relatively high DC voltage output of rapidly changing amplitude adapted to drive the color changing circuitry of a CRT and specifically the anode of a beam penetration CRT or similar low current (less than one milliamp) load.
As just stated, the voltage power supply disclosed herein is especially suitable for driving the anode of a beam penetration CRT. A voltage power supply utilized in this way is required to apply a rapidly changing voltage output which corresponds to and is ultimately responsible for producing changing colors in the CRT. There is however a problem in rapidly changing the anode voltage of a CRT. Specifically, the anode capacitance (typically on the order of 400 pF to 1000 pF) must be charged to increase voltage and discharged to decrease it. This charging and discharging action of a capacitor is described by the well known charge law: EQU .DELTA.V=.DELTA.Q/C (1)
.DELTA.V is the voltage change and .DELTA.Q is the amount of charge (in coulombs) to be added or removed.
In a "slewing" method of operation, that is, where the input voltage to the CRT anode is either pulled up or pulled down, charges are added or removed at an approximately constant rate so that equation 1 above may be rewritten as: EQU .DELTA.V=I.DELTA.T/C (2)
I is an approximately constant current and .DELTA.T is the time required to produce the voltage change .DELTA.V.
As an example of the foregoing, let it be assumed for the moment that .DELTA.V equals 8000 V, C=1000 pF and .DELTA.T=200 microseconds. Based on these values, I=40 milliampere. Each time a change .DELTA.V is produced, energy is added or removed from the capacitor according to: EQU E=1/2 C .DELTA.V.sup.2 (joules) (3)
If the change .DELTA.V is repeated N times each second then the flow of energy into the capacitor is given by: EQU W.sub.IN =(N C .DELTA.V.sup.2 /4) (watts) (4)
Similarly, the flow of energy out of the capacitor is: EQU W.sub.OUT =N C .DELTA.V.sup.2 /4 (5)
For example, if the change calculated above were to be repeated 500 times per second, W.sub.IN would equal 8 watts and W.sub.OUT would equal 8 watts. Thus, the circuit must be capable of sourcing and sinking a substantial current (in the 40 to 80 milliamp range) during the slewing interval and must also be capable of sourcing and sinking a substantial amount of power if a moderately high repetition rate is required.
As will be seen hereinafter, the voltage power supply of the present invention is one which is especially suitable for driving a CRT while at the same time meeting the various demands placed on it. As will also be seen, in the circuit approach of the present invention, specifically in its preferred embodiment, the power for the slewing function is 10 kv regulated with an output of up to 5 milliamp, i.e. a total power equal to 50 watts maximum. As will also be seen, the voltage power supply of the present invention includes a number of uncomplicated and yet advantageous circuit arrangement for providing an uncomplicated and yet reliable method of producing a relatively high DC voltage output of rapidly changing amplitude.